Considering changing rear end ratio
#31
The main thing I have found is because of the load on the engine wit0h 3.08 it is in open loop a lot which kills mileage, open loop for those that don't know means the pcm ignores most sensors and reverts back to a base program for fuel and timing usually way rich ignoring inputs from the o2 sensor to trim fuel ratio for mpg.
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Edit: As posted before- I have a 94 f150 302 rclb 4r70w with 3.08's. I just bought the truck in October. Its not my daily. I'm married to this fuel mileage for now but I don't love it,lol.I'm in SE Michigan where its as flat and as straight as it gets.Set your big toe and go,lol. This truck replaces a 95 f150 302 3.55 5 spd ext cab long bed. That I felt did fairly well with power and mpg. I'm looking to see what type of mpg I get towing a smaller pop up.
#32
The main thing I have found is because of the load on the engine wit0h 3.08 it is in open loop a lot which kills mileage, open loop for those that don't know means the pcm ignores most sensors and reverts back to a base program for fuel and timing usually way rich ignoring inputs from the o2 sensor to trim fuel ratio for mpg.
#33
On edit: Force of drag increases with the square of velocity. Meaning there's a big difference from 55 to 70 mph in wind resistance. It takes fuel to go fast in these old bricks, but a lighter (RCSB 2X4), lower brick, with the i6, on stock tires should do fairly well at 70mph.
#35
#36
open loop comes into play when throttle angle, map reading tell the pcm the engine load has reached a certain tipping point.Now exactly where that is programmed I'm not 100 percent sure,it also factors in engine temp, o2 temp,transmission gear etc. The reason they can do this is emission standards are mostly at idle and cruise speeds measured in ppm, grams per mile etc. Emissions are not tested under heavy throttle loads pretty much the same as epa mpg ratings weren't real world.
#37
Also in the bad old days of carbs and cats it was found out that prolonged heavy loads melted the catalyst and caused car fires. The solution they came up with and still use today is altho it sounds counterintuitive is add more fuel to cool the converter. The reason is the same as when your mixture is to rich an engine wont run it takes a certain amount of oxygen to burn the fuel, addin g extra displaces the oxygen so it cant ignite and it cools the catalyst. Sounds crazy don't it but its true. It works the same as the air pump injecting oxygen causing the catalyst to light off but in reverse.
#38
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Exact conditions for open loop is going to vary with each PCM family but the bottom line is it is based on a dynamic factor called Engine Load and that parameter takes into account engine rpm, throttle angle, manifold vacuum or MAF flow rate, vehicle speed and a few other things. The PCM will transition into open loop as the vehicle travels along when Engine Load increases beyond a set point that is somewhere around 50-60%.. I can't remember exactly right now. It doesn't take much to do this, accelerating at anything more than a very moderate level will do it, simply driving down the read at a constant speed and encountering a hill will do it, this is a big part of the reason that automatic transmissions are programmed to downshift so easily, the whole idea is to try to keep the engine in closed loop as much as possible for best fuel milage. I had a tuner on my '90 that would allow realtime data logging and this was something that could be monitored, it was painfully obvious that when cruising along at 55mpg on a rolling highway that it was far better to keep the trans in Drive vs OD, when encountering hills in OD the engine would bog, Engine Load increased and forced open loop and the instantaneous fuel milage that the software calculated would drop from 20+ to 8-12mpg.
#39
#40
And being that he is trying to push a barn door down the road at the speed he wants to drive, going to a gear with LESS mechanical advantage he will load the engine more. Thus causing vacuum to be lower and be more likely to drop out of closed loop.
His best bet to test whether or not a gear swap will help is to try to find a friend with the same bolt pattern and taller tires. And then go for a test drive.
His best bet to test whether or not a gear swap will help is to try to find a friend with the same bolt pattern and taller tires. And then go for a test drive.
#43
I say do it. I had a 300 Six in my 1991 originally and it had the 3.55 gears. I swapped the entire rear-end to a 3.08 hoping to improve MPG, which was not what I was told to expect from the Six (about 16 is what I got). The MPG stayed about the same but the engine was much happier with the taller gear. Y'all flame away ONLY IF you have actually done this swap yourself and are not just bench racing.
Agree 3.55 is better for the 302 and even my current 460 but the Six likes a tall gear in my experience.
You do understand that max torque for the Six is *at* 2000 RPM, right?
Agree 3.55 is better for the 302 and even my current 460 but the Six likes a tall gear in my experience.
You do understand that max torque for the Six is *at* 2000 RPM, right?
#44
For some closure on this topic, the $$$ I would have spent on gears, gaskets and gear oil went to putting new heads on my Ranger and having it back to being the main daily driver getting close to 20 MPG at 70 MPH. The F150 is the boat hauler and Home Depot trip truck. Towing the boat, I am fine with bad mileage and it actually does pretty good in the city cruising at 40 mostly.
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